Organic EL display apparatuses are known as a display apparatus characteristic of a thin structure, high image quality, and low power consumption. An active matrix organic EL display apparatus includes two-dimensionally arranged multiple pixel circuits, each pixel circuit including an organic EL element and a driving transistor. The organic EL element is a self-light-emitting electro-optical element, whose luminance varies in response to a driving current thereof. The driving transistor is connected in series with the organic EL element, and controls an amount of driving current flowing through the organic EL element in response to a voltage between a gate and a source thereof.
The driving transistor typically used in a pixel circuit is a thin film transistor (hereinafter referred to as TFT). More specifically, transistors as the driving transistor include an amorphous silicon TFT, a low-temperature poly-silicon TFT, an oxide TFT (also referred to as oxide semiconductor TFT), and the like. The oxide TFT includes a semiconductor layer of oxide semiconductor. The oxide TFT is manufactured of indium gallium zinc oxide (In—Ga—Zn—O).
The gain of a transistor is typically determined by a mobility, a channel width, a channel length, and a gate insulation film capacitance, and the like. An amount of current flowing through the transistor varies depending on a gate-source voltage, a gain, and a threshold voltage. If a TFT is used for the driving transistor, variations occur in the threshold voltage, the mobility, the channel width, the channel length, and the gate insulation film capacitance. If the characteristics of the driving transistor vary, variations occur in an amount of a driving current flowing through the organic EL element. For this reason, the luminance of the pixel also varies, degrading display quality.
Organic EL display apparatuses that compensate for variations in the characteristics of the driving transistor have been studied. Patent Literature 1 through 4 and Non-Patent Literature 1 disclose organic EL display apparatuses that compensate for variations in the threshold voltage only. Patent Literature 5 through 9 disclose organic EL display apparatuses that perform both the threshold voltage compensation and gain compensation (mobility compensation).